Method for forming and washing a fibrous web

ABSTRACT

A layer of fibrous material is formed from a suspension of the fibrous material in a suspension liquid by introducing the suspension into a web-forming space defined between a member impermeable to the suspension liquid and a member that is permeable to the suspension liquid but substantially impermeable to the fibrous material and that is moved along an endless path. The thickness of the web-forming space diminishes from the upstream end to the downstream end thereof, relative to the direction of movement of the permeable member. As the suspension moves through the web-forming space, liquid drains through the liquid permeable member under the influence of pressure developed by the delivery pressure, coupled with the convergence of the space, thereby producing a fibrous layer in the web-forming space. The fibrous layer is washed by supplying liquid under pressure through a permeable element positioned opposite the permeable member downstream from the web-forming space. The volumetric flow rate of liquid is controlled so that the wash liquid displaces the suspension liquid in the web to a depth that is less than the thickness of the layer. The remaining suspension liquid and a small amount of wash liquid are ultimately displaced from the web in a press nip.

United States Patent [191 Eriksson et al.

[ METHOD FOR FORMING AND WASHING A FIBROUS WEB [75] Inventors: Erik Sture Eriksson; Gosta Ingemar Ingemarsson, both of Karlstad, Sweden [73] Assignee: Aktiebolaget Karlstads Mekaniska Werkstad, Karlstad, Sweden [22] Filed: Dec. 11, 1972 211 App]. No.: 314,164

[30] Foreign Application Priority Data [56] References Cited UNITED STATES PATENTS 3,086,454 4/1963 Asplund 100/121 3,220,340 11/1965 Frykhult 100/121 3,342,124 9/1967 Frykhult 100/121 Primary Examiner-Harvey C. Hornsby Assistant Examiner-Philip R. Coe

Attorney, Agent, or Firm-Brumbaugh, Graves, Donohue & Raymond June 28, 1974 [5 7] ABSTRACT A layer of fibrous material is formed from a suspension of the fibrous material in a suspension liquid by introducing the suspension into a web-forming space defined between a member impermeable to the suspension liquid and a member that is permeable to the suspension liquid but substantially impermeable to the fibrous material and that is moved along an endless path. The thickness of the web-forming space diminishes from the upstream end to the downstream end thereof, relative to the direction of movement of the permeable member. As the suspension moves through the web-forming space, liquid drains through the liquid permeable member under the influence of pressure developed by the delivery pressure, coupled with the convergence of the space, thereby producinga fibrous layer in the web-forming space. The fibrous layer is washed by supplying liquid under pressure through a permeable element positioned opposite the permeable member downstream from the web-forming space. The volumetric flow rate of liquid is controlled so that the wash liquid displaces the suspension liquid in the web to a depth that is less than the thickness of the layer. The remaining suspension liquid and a small amount of wash liquid are ultimately displaced from the web in a press nip.

4 Claims, 1 Drawing Figure FATERTEMWQR HM METHOD FOR FORMING AND WASHING A FIBROUS WEB BACKGROUND OF THE INVENTION Cellulose is extracted from cellulosic materials, such as wood, by treating the cellulosic material with certain chemicals in a digester. The chemical digestion process produces a suspension of cellulosic fibers in a suspension liquid that contains various dissolved organic and inorganic substances. After digestion, the cellulosic fibers are separated from the suspension liquid and washed to remove,-as effectively as possible, the suspension liquid and the dissolved organic and inorganic substances that it contains. It is desirable to save the suspension liquid rather than discharging it as waste, since some of the chemicals in the suspension liquid can, to advantage, be recovered and reused in the cellulose extraction process. Moreover, some organic substances in the suspension are useful as fuel, and it is at least inappropriate, and it has been made unlawful in many areas, to discharge suspension liquid into water courses, since the suspension liquid contains pollutants.

Various processes for recovering useful substances from the suspension liquid involve evaporation techniques. Accordingly, the quantities of liquids used in the digestion process should be kept as small as possible. Similarly, it is desirable that the washing process be carried outwith as little washing liquid as possible, inasmuch as the washing liquid becomes mixed with and dilutes the suspension liquid.

Swedish Pat. No. 318,183 (see also corresponding US. Pat. No. 3,616,660) describes apparatus for separating the cellulosic fibrous material produced by a digestion process from the suspension liquid and for washing the cellulosic fibrous material. The apparatus described and illustrated in that patent includes a rotatable drum having a liquid permeable shell and two or more plate or shell-like pressing devices arranged in adjacent relation and defining with the shell a space, the thickness of which diminishes in the direction of rotation of the drum. A liquid permeable wire moves within the pressing elements, and the suspension of fibrous cellulosic material is supplied to the space between the wire loop and the drum at a location adjacent the upstream end of the first pressing element. As the suspension liquid flows through the convergent space defined between the wire and the drum, the liquid drains inwardly through the shell and outwardly through the wire and through the first pressing element, which is perforated or otherwise permeable to liquid, thereby forming a layer of the fibrous cellulosic material in sandwich relation between the shell and wire. The suspension liquid is collected both inside the drum and in a casing outside of the first pressing element and is removed for further processing. Near the end of the forming space between the drum and the wire, the layers of fibrous material formed on the surface of the drum and on the inside of the wire as suspension liquid is drained are combined into a single layer which is conveyed between the wire and the drum into the zone between the second pressing element and the drum.

A second zone, the space between the second pressing element and the drum, is a washing zone, and washing liquid is supplied under pressure through the second pressing element, which is perforated or otherwise liquid permeable, thereby to displace suspension liquid remaining in the fibrous layer carried between the drum and wire. The fibrous web then leaves the second pressing element and is conducted between the wire and the drum through a press nip in which the fibrous web is pressed to remove washing liquid. The web is then separated from the drum and wire and removed from the apparatus.

An important advantage of the apparatus describedand illustrated in Swedish Pat. No. 318,183 is that the fibrous web is maintained in a compressed condition during washing in the washing zone, such pressure resulting from the maintenance by the pressing element of a confined space of predetermined thickness through which the web moves in sandwiched relation between the wire and drum. Accordingly, after washing the fibrous web has relatively low liquid content, and both lumen liquid and volumes of liquid present in the spaces between the fibers of the cellulosic material participate in the displacement of suspension liquid by washing liquid. Accordingly, excellent washing is obtained.

SUMMARY OF THE INVENTION The present invention relates to an improvement of the apparatus described and illustrated in Swedish Pat. No. 318,183 and similar apparatus. More particularly, the apparatus of that patent is normally operated with washing liquid supplied under high pressure in the washing zone. Consequently, the washing liquid tends to penetrate entirely through the web along the major part of the length of the washing zone. As a result, a relatively large quantity of washing liquid is required.

In accordance with the present invention, the rate of supply of washing liquid and the pressure under which it is supplied are controlled so that the washing liquid does not at any point along the washing zone pass entirely through the layer of fibrous material but displaces the suspension liquid only to a controlled thickness or depth less than the total thickness of the web. Thus, while the apparatus of the present invention retains the advantages of the apparatus described in the Swedish Pat. No. 318,183, it provides the additional advantage of enabling washing to be accomplished with a substantially smaller amount of washing liquid. The apparatus of the present invention also embodies certain other modifications of the apparatus described in the aforementioned Swedish patent. The present invention also provides a method for separating fibrous material from a suspension liquid and for washing the fibrous material, as described more fully hereinafter.

More particularly, the apparatus, according to the invention, includes a member that is permeable to liquids and substantially impermeable to the fibrous material and is movable along an endless path. A member positioned opposite the permeable member defines with the permeable member a layer-forming space, the thickness of which diminishes from the upstream end of the space to the-downstream end, relative to the direction of movement of the permeable member. The suspension of fibrous material is supplied to the upstream end of the forming space. A liquid permeable element positioned opposite a portion of the permeable member immediately downstream from the downstream end of the layer-forming space defines with the permeable member a washing zone, and wash liquid is supplied under pressure through the permeable element to the washing zone. A press roll located downstream from the downstream end of the washing zone forms a press nip with the permeable member.

With the apparatus, as described thus far, the suspension liquid supplied to the layer-forming space is drained through the permeable member to form I a layer, the drainage being a result of forcing the fibrous suspension through the layer-forming space as a result of the diminishing of the thickness of the space and the consequent forced pressure drainage of liquid through the permeable member. The layer of material (or fibrous web) leaving the forming space passes through the wash zone where wash liquid is displaced into it and displaces suspension liquid.

In accordance with the invention, the volumetric flow rate of washing liquid supplied to the washing zone is controlled in accordance with the quantitative flow rate of fibrous material through the washing zone. Such control is indirect in the sense that it is obtained by monitoring and controlling other aspects of the operation of the equipment to ensure that the washed and pressed layer of fibrous material leaving the press nip is of substantially uniform dryness and thickness. Uniformity of dryness and thickness of the web upon leaving the press nip is obtainedif the linear pressure in the press nip and the thickness of the press nip are maintained constant. Under these conditions of uniform dry content and thickness, it is readily possible to compute the quantity of washing liquid to obtain a desired degree of displacement of the washing liquid into the fibrous web.

Preferably, the volumetric flow rate of wash liquid is controlled so that the wash liquid displaces the suspension liquid from the fibrous layer to a depth at the end of the washing zone of the apparatus that is less than the total thickness of the layer at the end of the washing zone. In other words, the volumetric flow rate of wash liquid is established to provide only partial displacement of the suspension liquid so that the wash liquid does not flow entirely through the fibrous layer at any point in the washing zone. Displacement of all of the suspension liquid from the layer, together with a small displacement of washing liquid from the layer is obtained in the press nip, the thickness of the press nip being a value that is less than the depth to which wash liquid penetrates into the web and displaces suspension liquid from the layer in the washing zone.

Maintenance of the desired washing effect, as described above, can be obtained by providing in the first instance a constant nip thickness between the press roll and the permeable member that carries the fibrous layer through the nip. A constant pressure in the nip so that the dryness of the layer as it leaves the nip is constant can be maintained by either controlling the speed of movement of the permeable member or controlling the volumetric rate at which fibrous suspension is supplied to the forming zone of the apparatus or by a combination of controlling the speed of movement of the permeable member and the rate of supply of fibrous suspension. For example, if fibrous suspension is supplied at a constant rate to the apparatus, but the penneable member moves at a variable speed, the amount of fibrous material conducted through the nip per unit time is varied in accordance with the speed of movement of the permeable member. The amount of material per unit time moving through the nip directly influences the pressure in the nip.

The volumetric rate of flow ofwash liquid in the washing zone is controlled in accordance with the volumetric rate of flow of the fibrous layer through the apparatus. Inasmuch as small variations will occur in the amount of fibrous material in the layer and thus the nip pressure will vary by small amounts from time to time, it is preferable that the wash liquid supply be varied in accordance with variations in the nip pressure. For example, indirect control of wash iquid supply in accordance with nip pressure is conveniently obtainable by monitoring either the speed of movement of the permeable member or the volumetric rate of flow of fibrous suspension to the layer-fonning zone or both.

Thus, the method and apparatus according to the invention, involve using a minimum amount of wash liquid only partially to displace the suspension liquid from the layer in the washing zone. Complete displacement of the washing liquid occurs in the press nip, the press nip having a thickness no greater than the depth to which was liquid is displaced into the layer in the washing zone, thereby ensuring that the remaining suspension liquid is displaced from the layer at the press nip. The invention ensures the maintenance of an appropriate volumetric rate of flow of wash liquid into the fibrous layer and the production of a washed and pressed layer of fibrous material of substantially uniform thickness, homogenous structure and a high dry content. The displacement of suspension liquid from the layer in the washing zone is obtained with a minimum of pressure difference across the layer so that disruption of the fibrous structure is held to a minimum. The layer reaches the washing zone at the end of the forming zone at a relatively high dry content, thereby minimizing the quantity of washing-liquid required to attain a desired washing effect. A uniform washing effect is assured, because the ratio of the volume of washing liquid supplied to the layer and the dry content of the web as it leaves the press nip are maintained constant.

Further features of the invention and the advantages provided by the invention may be better understood by reference to the following description of an exemplary embodiment, taken in conjunction with the figures of the accompanying drawings.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a generally schematic side-elevational view of an embodiment of the apparatus of the invention and depicting a method according to the invention; and

FIG. 2 is a side view of the washing zone and press nip of the apparatus of FIG. 1 illustrating the progress of wash liquid displacement of suspension liquid as the layer progresses through the wash zone and the press nip, the view being on a larger scale than FIG. 1.

DESCRIPTION OF EXEMPLARY EMBODIMENT In the embodiment of the invention illustrated in the drawings, the permeable member that is moved along an endless path is in the form of a rotatable drum I having a shell 3 that is permeable to the suspension and wash liquids but substantially impermeable to the fibrous material. The drum 1 is mounted for rotation in journals 5 and is driven by an appropriate drive system that includes a variable speed drive 7. A shell or casing 9 that extends around a large portion of the drum surface includes a portion that is impermeable and defines a closed layer-forming space, the thickness of which, as measured in the radial direction relative to the shell,

diminishes from the upstream end to its downstream end, relative to the direction of rotation of the shell (which direction is clockwise in the figures of the drawings, as indicated by the arrows).

The fibrous suspension to be formed into a layer and washed in the apparatus is delivered at the upstream end of the forming space 11, the supply apparatus being of a construction such that a substantially uniform flow of suspension is supplied along the whole length of the upstream end of the forming zone 11. One or more controllable valves or gates 13 is provided to enable the volumetric flow rate of the fibrous suspension to the forming zone to be controlled.

A perforated or otherwise liquid permeable portion of the shell 9 extends from the downstream end of the impermeable portion and defines with the zone of the drum surface over which it extends a washing zone 19. Preferably, the thickness of the washing zone (measured radially of the drum) diminishes in the downstream direction and terminates at the downstream end in a slot or opening 15 of substantially uniform thickness. A casing 23 associated with the permeable portion of the shell forms a chamber for the supply of wash liquid to the washing zone 19, and wash liquid is conducted from a supply 25 through one or more variable flow control valves 27 of the type that enables the volumetric flow rate of wash liquid to the washing zone 19 to be controlled. The wash liquid passes from the chamber through perforations 21 into the web.

A rotatable press roll 29 located immediately down- .stream from the downstream end of the washing zone 19 defines with the drum surface a press nip 35, the surface or shell 31 of the press roll being liquidimpermeable. The press roll 19 is mounted in journals 33, preferably in the same machine framework as the journals for the drum 1, so that the thickness of the press nip may be maintained substantially constant during operation. A pressure monitor 37 for continuously monitoring the pressure in the press nip 35, such as a strain gauge type of pressure measuring instrument, is associated with the press roll journals 33 and produces a signal indicative of the linear pressure in the press nip 35. Apparatus for removing the web of fibrous material after it leaves the press nip is provided, such apparatus including a doctor blade 39 and a combined feeding and shredding screw 41.

The operation of the apparatus of the invention is controlled by a control system comprising a controller 43 that is connected by control signal conductors 45, 47 and 49 to, respectively, the press roll pressure monitor 37, the speed control device 7 and the controllable valve or valves 27 associated with the washing liquid supply system. The specific type and construction of the components of the control system and the system as a whole may be selected, as a matter of ordinary engineering skill, by one skilled in the art from a variety of known electrical, hydraulic, pneumatic or mechanical components and systems that are well known per se.

- the valve or valves 13 being appropriately set to provide the desired flow rate. The drum 1 is rotated at a substantially constant speed of a desired, appropriate value. The pressure of the fibrous suspension delivered to the forming space is above the pressure withinthe drum, and consequently suspension liquid 53 passes through the liquid permeable shell 3 of the drum and is removed from within the drum through an outlet 50. A fibrous web 55 is progressively formed as drainage of liquid through the permeable shell occurs throughout the web-forming space 17. In particular, as illustrated schematically in FIG. 1 of the drawings, the thickness of the fibrous web gradually increases as the web moves with the rotating drum through the web-forming space until the thickness of the web is equal to the thickness of the space between the shell 3 and the casing 9 and thus fills the space. After the web builds to the thickness equal to the thickness of the forming space, it is thereafter subjected to mechanical compression as it continues to move with the rotating drum through the downstream end portion of the web-forming space and through the washing zone 19. Liquid continues to drain from the web due to the mechanical pressure, but the pressure is maintained by the convergence of the casing 9 toward the shell 3 of the drum.

As the fibrous web 55 moves through the washing zone 19 of the apparatus, washing liquid 57 is delivered at a generally constant but controlled quantity per unit of time, i.e., volumetric flow rate, under pressure into the chamber 23 and passes through the perforations 21 in the perforated portion of the casing and flows into the fibrous web. The flow of washing liquid under pressure into the web displaces suspension liquid 53 remaining in the fibrous web to a degree that is a function of the flow rate of liquid into the web. .As illustrated'in FIG. 2, such displacement occurs continuously and progressively as the web 55 moves through the washing zone, the degree or level of penetration of washing liquid into the web and consequent displacement of suspension liquid from the web depicted by the line AA. In other words, above the line AA in FIG. 2, washing liquid has displaced suspension liquid from the web, while below the line AA in FIG. 2, suspension liquid remains in the web, washing liquid having not penetrated into the web below the line AA.

The web passes from the washing zone into the press nip 35 where it is pressed to a thickness substantially less than its thickness as it left the washing zone through the opening or slot 15 at the downstream end of the washing zone. Preferably, the pressure in the press nip is such as to produce a web that leaves the press nip with a dryness of in the range of from 30 to percent. Inasmuch as the shell 31 of the press roll 29 is liquid impermeable, all drainage occurring in the press nip is through the shell 3 of the drum 1. Consequently, the first fraction of the liquid content of the web to drain through the shell 3 is the suspension liquid 53 that remains in the fibrous web 55 as it leaves the washing zone. Subsequently a part of the washing liquid 57 is also pressed from the web and through the permeable shell 3. It is evident from the foregoing, and also from a consideration of FIG. 2 of the drawings that the only place in the apparatus where wash liquid actually is drained or pressed from the fibrous web is in the press nip. Preferably the amount of washing liquid removed from the web at the press nip is kept to a minimum, thereby to minimize the amount of washing liq uid required in the processing and thus to limit the amount of liquid ultimately to be treated for recovery of valuable chemicals and to condition it for discharge from the system. 7

The maintenance of a flow rate of wash liquid into the washing zone to produce a desired profile in the extent of displacement of wash liquid to the level approximating the line AA in FIG. 2 of the drawings requires coordinating several variables employing the control system of the apparatus, such variables including the linear pressure in the press nip 35, the speed of rotation of the drum 1 and the flow rate of washing liquid 57 into the casing. Preferably, the thickness of the press nip 35 and the rate of supply of fibrous suspension 51 are maintained essentially constant. Appropriate control devices known per se and not shown in the figures of the drawings are employed to maintain a constant press nip thickness and a constant flow rate of fibrous suspension to the apparatus.

The pressure monitor 37 associated with the press roll continuously produces signals indicative of the actual value of the linear pressure in the press nip 35, and those signals are conducted to the controller 43 through conductors 45. The controller includes a device for generating a signal indicative of a known, desired pressure value, which is hereafter referred to as a set point value. The controller further includes a device or devices for continuously monitoring, and for detecting any difference between, the actual value of linear pressure derived from the pressure monitor and the set point value. Upon detecting a difference between the actual linear pressure and the set point value, the control unit produces a signal indicative of such difference, and such signal is processed and conducted as a correction signal to the speed control device 7 of the druml. The correction signal conducted to the speed control device 7 is such that when the linear pressure drops below the set point value in the controller, the speed of the drum will be reduced, thereby slowing the movement of the drum, allowing a thicker web to form for a given supply of fibrous suspension, and ultimately increasing nip pressure. Conversely, if the correction signal is indicative of an actual linear pressure in the press nip above the set point value, the speed of the drum 1 is increased, with an effect that is the reverse of the one just referred to for a decrease in drum speed.

The controller 43 also produces a control signal that is a function of the actual value of the speed of the drum, as monitored by a monitor portion of the speed control element 7, and transmits that signal to the servo-controlled valve or valves 27. The servo-valve adjusts the flow rate of washing liquid to the washing zone of the apparatus in accordance with the flow rate of fibrous material carried by the drum through the washing zone.

The above-described embodiment of the invention is intended to be merely exemplary, and many variations and modifications of it will be readily apparent to those skilled in the art without departing from the spirit and scope of the invention. All such variations and modifications are intended to be included within the scope of the invention as defined in the appended claims.

We claim:

1. In a method for forming a layer of fibrous material from a suspension of the fibrous material in a suspension liquid and for washing the layer with a wash liquid, the method including the formation on a moving liquidperrneable member of a layer of the fibrous material, the displacement of suspension liquid from the layer by introducing a wash liquid into the layer from one side thereof and then depressing liquid from the layer in a press nip, the improvement comprising the steps of controlling the volumetric flow rate of wash liquid into the fibrous layer such that the wash liquid displaces the suspension liquid in the fibrous layer to a depth less than the thickness of the layer at the region along a path of movement of the layer where wash liquid ceases to be introduced into the layer and pressing the layer in the press nip to an extent such that the minimum thickness of the layer in the nip is not in excess of the maximum depth to which wash liquid displaces suspension liquid from the layer in the washing step.

2. The improvement according to claim 1 and further comprising the step of monitoring the linear pressure in the layer at the press nip and producing a nip-pressure signal indicative thereof, varying the volumetric rate of flow of fibrous material through the nip in response to the nip-pressure signal, monitoring the volumetric rate of flow of fibrous material through the nip and producing a signal indicative thereof, and controlling the volumetric flow rate of washing liquid to the washing zone in response to said signal.

3. The improvement according to claim 2 wherein the volumetric flow rate of fibrous material through the nip is varied by varying the speed of movement of the layer of fibrous material through the nip, the suspension of fibrous material being delivered to the moving liquid-permeable member at a substantially constant rate.

4. The improvement according to claim 2 wherein the volumetric rate of flow of fibrous material through the press nip is varied by varying the rate of supply of the suspension of the fibrous material to the moving liquid-permeable member, the permeable member being moved at a substantially constant speed. 

1. In a method for forming a layer of fibrous material from a suspension of the fibrous material in a suspension liquid and for washing the layer with a wash liquid, the method including the formation on a moving liquid-permeable member of a layer of the fibrous material, the displacement of suspension liquid from the layer by introducing a wash liquid into the layer from one side thereof and then depressing liquid from the layer in a press nip, the improvement comprising the steps of controlling the volumetric flow rate of wash liquid into the fibrous layer such that the wash liquid displaces the suspension liquid in the fibrous layer to a depth less than the thickness of the layer at the region along a path of movement of the layer where wash liquid ceases to be introduced into the layer and pressing the layer in the press nip to an extent such that the minimum thickness of the layer in the nip is not in excess of the maximum depth to which wash liquid displaces suspension liquid from the layer in the washing step.
 2. The improvement according to claim 1 and further comprising the step of monitoring the linear pressure in the layer at the press nip and producing a nip-pressure signal indicative thereof, varying the volumetric rate of flow of fibrous material through the nip in response to the nip-pressure signal, monitoring the volumetric rate of flow of fibrous material through the nip and producing a signal indicative thereof, and controlling the volumetric flow rate of washing liquid to the washing zone in response to said signal.
 3. The improvement according to claim 2 wherein the volumetric flow rate of fibrous material through the nip is varied by varying the speed of movement of the layer of fibrous material through the nip, the suspension of fibrous material being delivered to the moving liquid-permeable member at a substantially constant rate.
 4. The improvement according to claim 2 wherein the volumetric rate of flow of fibrous material through the press nip is varied by varying the rate of supply oF the suspension of the fibrous material to the moving liquid-permeable member, the permeable member being moved at a substantially constant speed. 